I haven't used MODX Evolution for years, so I'm limited in how much I can help.
I don't know if eForm has a way for you to capture the form data after it's submitted or not, but even if it does, you'd need some custom PHP code to handle it.
I tend to write my own form handling PHP code rather than using form extras. You could start with some articles about PHP form handling:
There's a PHP primer in the Appendix of my book
that I think is a really good place for beginners to start learning PHP, and the book is full of code examples showing how to do things in MODX. The book leans toward MODX Revolution (which I recommend that you switch to if you are just starting the site), but there's a fair amount of Evolution stuff in there too. The MODX Manager section is out-of-date because the Manager has been reorganized, but most people find things without too much trouble.
There are also plenty of free PHP tutorials on the web: https://www.google.com/search?q=learning+php&oq=learning+php